Method of plating chromium over antimony



IVIETHOD 01F PLATING CHROMIUM OVER ANTIMONY Clarence F. Smart, Birmingham, Mich, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware 5 C. (Cl. 204-29) This invention relates to improvementsin the electrodeposition of metals and particularly to the electrodeposition of chromium over antimony or antimony alloys.

Chromium is one of the more difiicult metals to electrodeposit satisfactorily. Chromium is conventionally electrodeposited from a hot aqueous electroplating bath contaming chromic and sulfuric acid. Such a bath is both corrosive and very oxidizing at the temperatures at which it is usually employed. Many metals, when immersed in such an acid solution, are corroded or etched to produce a rough, uneven surface, poorly suited as an undercoat for a bright electrodeposit of chromium. For this reason, in electrodepositing chromium over a surface of antimony or an antimony alloy, to obtain an adherent coating of chromium, it has been considered essential not to expose the antimony or antimony alloy surface to the corrosive action of the hot chromic acid plating bath. Thus in plating chromium over antimony or alloys of antimony it has heretofore been thought necessary to apply the plating current as the article to be plated is immersed in the chromium plating bath so that electrodeposition of chromium would begin immediately.

I have now discovered that a bright, adherent, corrosion resistant coating of chromium may be electrodeposited over an article having 'an outer surface of antimony or of an antimony base alloy if, instead of immediately applying the plating current when the article to be plated is immersed in the chromium plating bath, the application of th plating current is delayed for a predetermined time.

In general, the practice of my invention involves exposing an article, having an outer surface of antimony or an antimony base alloy, upon which chromium is to be electrodeposited, to a chromium plating bath for a period of time not exceeding sixty seconds before any plating current is applied to the bath and then electrodepositing chromium on the article by passing a plating current through the bath. In a preferred embodiment, the article is immersed in a hot chromium plating bath and the delay in the application of the plating current is from about five to thirty seconds.

It is not clearly understood just why such a treatment of the antimony or antimony base alloy surface of the article to be plated should produce such a singular improvement in the brightness of the electrodeposited chromium. Compared to other metals conventionally chromium plated, which would be readily corroded and etched by such a treatment, antimony or antimony base alloys are generally quite resistant to the corrosive action of the hot chromium plating solution. The beneficial results obtained do not seem to depend on any electric cell action in the bath since it seems to be immaterial whether the article to be'plated is immersed in the bath while connected in the plating circuit or is merely immersed in a separately maintained chromium plating bath while not connected in the circuit. It is thought that during the period in which the antimony or antimony coated article is immersed in the hot chromium plating bath, its

2,764,538 Patented Sept. 25, 1956 surface is chemically cleaned and a very thin surface film is formed on the article which, when the current is later applied to the bath, permits the electrodeposited chromium to more firmly adhere to the cathodic antimony containing surface forming 'a brighter, denser coating. It Will be understood, of course, that my invention should not be limited by any theoretical explanation attempted herein as to why such beneficial results are obtained.

In ele'ctrodepositing a bright, adherent, chromium coating over a surface of antimony or antimony alloy according to the method of my invention, any suitable chromic acid chromium plating bath may be used. The bath may contain, for example, from about 33 to 53 ounces per gallon of chromic acid (CrOs) and a small amount of sulfuric acid. The ratio of chromic acid to sulfuric acid may of course be varied. As a specific example, an aqueous chromium plating bath containing 53 ounces per gallon of chromic acid (OrOs) and 0.53 ounces per gallon of surfuric acid may be used. Such a plating bath may be operated at temperatures ranging from F. up to about 160 F. although it preferably is operated at a temperature Within a range of from F. to F. According to the method of my invention, such a chromium plating bath generally may be operated at a current density within the range of about to 265 amperes per square foot. However, most satisfactory results are obtained when the bath is operated at a current density of about to 220 amperes per square foot.

While a bright layer of chromium may be deposited according to the method of my invention in any conventionally applied thickness, for example, up to about 0.0001 inch if desired, in a preferred embodiment, most satisfactory results are obtained when the bright chromium coating deposited over the antimony or antimony base alloy has a thickness of from 0.00001 to 0.000015 inch. F or example, a flash coating of chromium, having a thickness of about 0.00001 inch, deposited over a bright antimony coated article provides a bright surface which is both corrosion and abrasion resistant.

While the method of my invention produces chromium coatings over antimony surfaces, it is equally applicable in the chromium plating of alloys of antimony in which antimony is the major constituent. Illustrative of such alloys which may be chromium plated are antimony-lead, antimony-copper, antimony-iron, antimony-cadmium, antimony-zinc and antimony-tin alloys.

Instead of electrodepositing chromium on an antimony coated article in the same chromium bath in which it is treated according to the method of my invention, if desired, the article to be plated may be exposed separately to a first hot chromium plating bath through which no plating current is passed and then chromium plated using a second :chromium plating bath where the full plating current is applied when the article is immersed in the bath. Hence, the method of my invention is readily adaptable for use in commercial electroplating procedures which employ a continuous conveyor system. For example, an article to be chromium plated may be moved by an automatic conveyor system through the entire cleaning, treating and plating sequence. in such an application the article may be carried through a conventional electrocleaning bath, a rinse bath and then immersed in a hot chromic acid bath for a period not exceeding 60 seconds before it is moved by the conveyor to a second hot chromic acid bath throughwhich a plating current is passed and chromium is electrodepo-sited. In this embodiment of my invention it is preferred that the article, after it has been immersed in the first chromic acid bath for the desired period of time, not be rinsed before it is immersed in the second bath from which chromium is plated so that any surface film formed during the treatment will not be removed. The chromic acid bath actually used for electrodepositing the chromium will not be diluted or contaminated by the rinse fluid dragged in by the article to be plated.

As an illustrative example of the production of a bright chromium coating according to the method of my invention, a metallic article, of iron, steel, copper or zinc for example, may first be electroplated with a smooth thin coating of lead, indium or a lead-indium alloy. This coatingv serves to protect the article being plated from attack of'any subsequent acid plating baths. The article may then be electroplated with antimony or an antimony base alloy. Since an electroplated coating mirrors the surface of its own undercoat, it is preferable to initially form and to maintain a bright surface on the article tobe chromium plated. Thus the antimony or antimony alloy surface serving as the chromium undencoat may be buffed or polished to achieve a bright, lustrous surface. By proper procedures and plating technique, however, a bright antimony or antimony alloy surface may also be obtained without the necessity of bufling or polishing. Then in accordance with a preferred embodiment of my invention the article is immersed in a chromic acid chromium plating bath having a temperature of about 115 F. to 130 F. and maintained there for about seconds before any plating current is applied to the bath. At the end of about 10 seconds, the full current, preferably a current sufhcient to provide a cathodic current density of about 180' to 220 amperes per square foot is applied. The article is then maintained in the chromium plating bath for a length of time suflicient to plate th desired thickness of chromium which, in a preferred embodiment, does not exceed about 0.000015 inch.

While my invention has been described specifically herein, it will be understood that the description is intended to be illustrative only and that my invention is to be measured only by the following claims in which it is my intention-to claim all novelty inherent in the invention.

I claim:

1.. A method of electrodepositing a bright coating of chromium on an article having an outer surface of a metal selected from the class consisting of antimony and antimony base alloys which comprises immersing said article for a period of time not in excess of one minute in a chromic acid base chromium plating bath maintained at a temperature of from110 F. to 160 F. while no electrical current is passing through the bath and then electrodepositing bright chromium on the articl in a chromic acid base chromium plating bath.

2. A method of electrodepositing a bright coating of chromium on an article having an outer surface of a metal selected from the class consisting of antimony and antimony base alloys which comprises immersing said article for a period of time not in excess of one minute in a chromic acid base chromium plating bath maintained at a temperature of from 115 F. to 130 F. while no electrical current is passing through the bath and then electrodepositing bright chromium on the article in a chromic acid base chromium plating bath.

3. A method of electrodepositing a bright coating of chromium on an article having an outer surface of a metal selected from the class consisting of antimony and antimony base alloys which comprises immersing said article for a period of time within the range of about 5 to 30 seconds in a chromic acid base chromium plating bath maintained at a temperature of from F. to 160 F. while no electrical current is passing through the bath and then electrodepositing bright chromium on the article in a chromic acid bas chromium plating bath.

4. A method of electrod'epositing a bright coating of chromium on an article having an outer surface of antimony which comprises immersing said article for a period of time not in excess'of one minute in achromic acid base chromium plating bath maintained at a temperature of from 110 F. to 160 F. while no electrical current is passing through the bath and then electrodepositing bright chromium on the antimony in a chromic acid base chromium plating bath, said bright chromium electrodeposit having a thickness not in excess of 0.0001 inch.

5. A method of electrodepositing a bright coating of chromium on an article having an outer surface of antimony which comprises immersing said article for a period of time within the range of 5 to 30 seconds in a chromic acid base chromium plating bath containing about 33 to 53 ounces of chromic acid per gallon while no electrical current is passing through the bath, said bath being maintained at a temperature of from F. to about F. and then electrodepositing bright chromium on the antimony in a chromium plating bath containing about 33 to 53 ounces of chromic acid per gallon, said bright chromium having a thickness within the range of 0.00001 to 0.000015 inch.

References Cited in the file of this patent UNITED STATES PATENTS Smith Dec. 8, 1925 Brown et al Aug. 23, 1938 OTHER REFERENCES 

1. A METHOD OF ELECTRODEPOSITING A BRIGHT COATING OF CHROMUIM ON AN ARTICLE HAVING AN OUTER SURFACE OF A METAL SELETED FROM THE CLASS CONSISTING OF ANTIMONY AND ANITMONY BASE ALLOYS WHICH COMPRISES IMMERSING SAID ARTICLE FOR A PERIOD OF TIME NOT IN EXCESS OF ONE MINUTE IN A CHROMIC ACID BASE CHROMIUM PLATING BATH MAINTAINED AT A TEMPERATURE OF FROM 110* F. TO 160* F. WHILE NO ELECTRICAL CURRENT IS PASSING THROUGH THE BATH AND THEN ELECTRODEPOSITING BRIGHT CHROMIUM ON THE ARTICLE IN A CHROMIC ACID BASE CHROMIUM PLATING BATH. 